The present invention relates to protecting ear-plugs of the kind comprising a plug-like body of fibrous material, preferably mineral fiber material, and especially so-called fine glass wool, and a casing which surrounds at least that part of the body which is intended to be inserted into the auditory canal.
A plurality of different ear-plugs of the kind mentioned hereinbefore have already been proposed. The surrounding casing in these earlier cases has been stated to be for example in the form of a thread net, a cotton weave or gauze, preshaped rubber, applied latex or a waxy binder sprayed on in a modelled fashion, and its function is to fix the plug and/or confine the fibrous material, as well as in certain cases to contribute to some extent to an increased attenuation by means of its own inherent sound attenuating effect.
One object of the present invention is to provide an improved ear-plug of the above mentioned kind, which without impairing the degree of comfort it affords when used gives substantially improved sound-attenuation, prevents fibre fragments from being left behind in the auditory canal when the plug is removed after use, and is easy to fix in place. The above object is achieved by virtue of the fact that the ear-plug in accordance with the invention exhibits the features set out in the attached claims.
The ear-plug in accordance with the invention is accordingly characterized substantially in that the casing comprises an external layer giving a substantially lower friction than the fibrous material in the plug-like body. The layer can be completely impervious but since it has been found that this may be experienced as a comfort impairment, it is advantageous to design the layer so that it permits pressure equalization at the time of insertion of the plug into the auditory canal and, possibly, also so that a certain degree of moisture penetration can take place after insertion, although these effects should not mean that any significant change takes place in the frictional properties of the layer. In accordance with the invention, these effects can be achieved in a simple and advantageous manner by arranging that at least that part of the layer which is intended to be introduced into the auditory canal, is provided with cuts permitting the development of passages through the layer as a consequence of differences in pressure between its two sides. The cuts suitably are so arranged that they delimit flaps, tongues or the like in the layer, which, at any increase in pressure between the plug and the ear-drum membrane, associated with the introduction of the plug into the auditory canal, can swing away so that temporary passages are created through which pressure equalization can take place. Moisture in the auditory canal can furthermore diffuse more quickly through the cuts. The friction between the layer and the wall of the auditory canal, in association with the introduction of the plug into the canal, is not afffected by this arrangment since the material in the plug-like body does not come into contact with the auditory canal.
Tests on volunteers have shown that an ear-plug provided with a casing in accordance with the invention surprisingly exhibits a typical increase in acoustic attenuation (in relation to a conventional glass wool plug, not provided with such an envelope but otherwise of identical design) of the order of 7 to 10 dB, which should be with a typical frequency-dependent attenuation of between around 8 and 26 dB for the conventional plug (attenuation measured in accordance with the hearing threshold method).
The reason for the improved acoustic attenuation can be attributed as follows:
To achieve a certain attenuation the plug-like body must contain a certain quantity of material. Furthermore, it is the quantity of material which is contained in the auditory canal, which is decisive in determining the attenuation. By providing the plug-like body with an external surface which has low friction in relation to the auditory canal, the introduction of the plug into the canal is simplified and this means that more material enters the auditory canal with a resultant improvement in sound attenuation. It should be emphasised that in tests the casing has not been found to produce any appreciable inherent attenuation. This has also been confirmed in tests on an artificial auditory canal, in which ear-plugs both with and without an external layer have been inserted for equal distances into the auditory canal without any appreciable difference in sound attenuation being observed.
The lower friction layer can advantageously be made of a thin plastic sheet or foil, for example of the adhesive film kind. Most thermoplastic sheets are suitable for use as the material of the layer. Advantageously polyethylene foil can be used since foil of this kind can be obtained in very thin qualities, is easily weldable, is entirely inert and does not give rise to any allergic reactions. The welding property means that the plug in accordance with the invention can readily be given a fixed shape for example by the use of a circumferential thermal weld in the plastic foil.